Powder metallurgy methods have been used as methods that can provide metal parts in any shape, and yield a metallurgical product primarily by mixing a lubricant into metal particles (powder) to give a metallurgical starting material, press molding this metallurgical starting material into a molding, and sintering this molding. A high-density or low-density metallurgical product may be obtained from press molding depending on the pressure, etc., but there has been strong demand in recent years for lighter metal parts and the demand for low-density metallurgical products has thus been increasing.
Patent Document 1 describes a metal powder mixture for powder metallurgy that is obtained by mixing polyethylene glycol, polypropylene glycol, glycerol, or polyvinyl alcohol as a binder into an iron or steel powder. Patent Document 2 describes a metal powder mixture for powder metallurgy that is obtained by mechanically mixing a binder selected from the group consisting of vinyl acetate copolymers, cellulose ester resins, methacrylic resins, alkyd resins, polyurethane resins, and polyester resins, into powder for alloying. Patent Document 3 describes a metal powder mixture for powder metallurgy that is based on ferrous powder and that contains a polyalkylene oxide having a number-average molecular weight of at least approximately 7,000 as a binder. However, all of these provide high-density green bodies (the molding prior to sintering) that have relative densities with reference to the ingot material in excess of 90% (represented by the “compression ratio” in Patent Document 1, the “unsintered density” in Patent Document 2, and the “green density” in Patent Document 3) and cannot provide low-density green bodies having low rattler values.
On the other hand, metal particles provided by granulation using agar are used in Patent Document 4, but this cannot provide a green body that realizes a low rattler value without inhibition of the lubricity and that is free of cracking, chipping, and density imbalances. Patent Document 5 describes a lubricating binder for low-density powder metallurgy, which uses an oxyalkylene polymer chain-containing polymer as a binder. However, the rattler values specifically reported in the examples of Patent Document 5 are from 3.8 to 4.5 (refer to Examples 8 to 15), and green bodies free of cracking, chipping, and density imbalances cannot be obtained with such rattler values at an actual facility for producing low-density powder metallurgy parts. There is thus desire for the realization of even lower rattler values at production facilities.    Patent Document 1: Japanese Patent Application Laid-open No. 56-136901    Patent Document 2: Japanese Patent Application Laid-open No. 63-103001    Patent Document 3: Japanese Patent Application Laid-open No. 6-10001    Patent Document 4: Japanese Patent Application Laid-open No. 2003-293001    Patent Document 5: Japanese Patent Application Laid-open No. 2005-330557